The present invention relates to a base station apparatus for use in a digital radio communication system that performs directional transmission by an adaptive array antenna in a forward link, and relates to a transmitting method.
The digital radio communication system such as a cellular phone, a car phone, and the like uses a cellular system in which a communication area is divided into cells, one base station apparatus is placed at each cell, and a plurality of terminal apparatuses performs broadcast communication with this base station apparatus. In recent years, since the cellular systems have spread rapidly and the number of system users has rapidly increased, improvement of increasing a channel capacity in one cell has become an important subject.
As a technique for increasing the channel capacity, an adaptive array antenna is described in xe2x80x9cWaveform Equalizing Technology for Digital Mobile Communications (supervised by Jun Horikoshi, Triceps). This is the technique in which the adaptive array antenna adds amplitude and phase shift to the antenna outputs of the array antenna composed of a plurality of antennas and combines them to form the directivity of array, whereby the adaptive array antenna determines a weighting factor of each antenna output based on a predetermined control algorithm, and controls the directivity as adapting to a change in an environmental state.
As a technique for improving in an increase of channel capacity, there is an adaptive array antenna described in such as xe2x80x9cWaveform Equalizing Technology for Digital Mobile Communications (supervised by Jun Horikoshi, Triceps). This is the technique in which the adaptive array antenna adds amplitude and phase shift to the antenna outputs of the array antenna composed of a plurality of antennas and combines them to form the directivity of array, whereby the adaptive array antenna determines a weighting factor of each antenna output based on a predetermined control algorithm, and controls the directivity as adapting to a change in an environmental state.
Then, the adaptive array antenna is mounted on the base station apparatus and interference from the other cell is canceled to improve a reception quality, and this makes it possible to improve an increase in the channel capacity of a reverse link.
In addition, the base station apparatus on which the adaptive array antenna is mounted estimates an arrival direction of a signal transmitted from the terminal apparatus, and performs directional transmission in the estimated direction, so that transmission power from the base station apparatus can be lessened as maintaining a link quality. This makes it possible to reduce interference with the other terminal apparatus and to improve the increase in the channel capacity of the forward link.
However, the base station apparatus estimates the signal arrival direction with respect to all terminal apparatuses with which the base station apparatus performs communications in the forward link, and carries out directional transmission, and this increases an amount of signal processing so as to cause a problem in which the apparatus size increases.
In order to solve such a problem, the convenional base station apparatus performs directional transmission with respect to some of the terminal apparatuses such as the terminal apparatus, which performs high-speed data transmission, or the terminal apparatus, which needs large transmission power by use of the array antenna. Then, the conventional base station apparatus performs transmission having no directivity such as sector transmission, omnidirectional transmission (hereinafter referred to as xe2x80x9cnondirectional transmissionxe2x80x9d) with respect to the other terminal apparatuses by use of an antenna prepared separately from the array antenna or one of antennas included in the array antenna.
This improves the increase in the channel capacity without increasing the apparatus size of a signal processing section so much.
However, since the aforementioned conventional base station apparatus combines a plurality of signals subjected to nondirecitonal transmission and transmits the combined signal from one antenna, transmission power in this antenna is not reduced. This causes a problem in which a dynamic range at an amplifying operation time of a radio section cannot be suppressed, so that a large-sized and large-valued amplification must be used.
It is an object of the present invention is to provide a base station apparatus, which can suppress necessary power for an amplifier and which can miniaturize the size of the apparatus in performing directional transmission with respect to only some of terminal apparatuses, and to provide the transmission method.
This object can be achieved by dividing signals subjected to directional transmission into signal components whose number is the same as that of antenna elements of an array antenna, respectively, assigning weights to the respective signal components of the divided signals subjected to directional transmission, allocating a signal subjected to no variable directional transmission to any one of the antenna elements, and combining signals transmitted from the same antenna element so as to perform radio transmission.